The first objective of this research proposal is to test a model of human pattern vision mechanisms with psychophysical results from a contrast matching task. A high-contrast background lowers the apparent contrast of a texture patch. A model of neural contrast normalization may explain this illusion, known as simultaneous contrast-contrast. The model posits that each cell in primary visual cortex is suppressed by the pooled activity of a large number of other cells. If perceived contrast depends on responses from these cortical cells, the normalization model predicts that background contrast should have a divisive effect on the perceived contrast of the central patch. The second objective is to search for the neural substrate of contrast perception. Recently researchers at several institutions have begun using a new kind of brain imaging technique, functional magnetic resonance imaging (fMRI) which, when coupled with a perceptual matching paradigm can bridge the gap to link perceptual appearance with brain activity. Neurons in the posterior calcarine have receptive fields in the central visual field. If contrast appearance is mediated by cells in primary visual cortex, then the fMR signals will reflect the changes in perceived contrast induced by the simultaneous contrast illusion.